Electrical and chemical measurements of sodium, potassium, chloride and hydrogen ion movements across the plasma membrane of an Aplysia giant neuron are being performed. Standard techniques of voltage- clamping and atomic absorption spectrophotometry are combined with more specialized techniques involving ion-selective microelectrodes. The nature of the light response in this neuron is also being examined. The phototransduction event is mediated by the release of calcium from cytoplasmic pigmented granules. The increased cytoplasmic calcium produces potassium conductance in the plasma membrane which hyperpolarizes the neuron. We have used electron microscopy and an electron microprobe to localize intracellular calcium and a microspectrophotometer to perform absorbance studies on the granules. The absorbance spectrum for the granules and the electrophysiological action spectrum are similar and the pigment involved is either trans beta carotene or retinal. In the latter case the linkage to the protein results in an absorbance peak at shorter wave lengths than that seen for rhodopsin. A model is being tested which would account for the quantitaive aspects of the response.